1. Field of the Invention
The invention relates generally to diagnostic instrumentation for semiconductor wafer processing equipment and, more specifically, to a capacitive probe for in situ measurement of the DC bias voltage accumulated on a semiconductor wafer while being processed in a semiconductor wafer processing system.
2. Description of the Background Art
When monitoring semiconductor wafer processing systems, it is important to accurately measure the DC bias voltage on the wafer. One method of measuring this value is with DC bias xe2x80x9cplugsxe2x80x9d. For example, in the P5000 metal etch chamber, manufactured by Applied Materials of Santa Clara, Calif., DC pickups or xe2x80x9cplugsxe2x80x9d composed of a graphite or solid silicon carbide shaft are placed in direct contact with a plasma. Assuming a near zero potential drop across the plasma, it is possible to estimate the DC bias on the wafer. Unfortunately, these types of xe2x80x9cplugsxe2x80x9d degrade and represent a consumable material that increases the maintenance costs of the chamber. Additionally, as the xe2x80x9cplugsxe2x80x9d are consumed, contaminants are released into the processing environment. As such, replacing the xe2x80x9cplugsxe2x80x9d with a device that measures DC bias voltage without directly contacting the plasma is preferred.
An improved measurement technique is to measure the amplitude of the RF voltage (e.g., peak-to-peak voltage) at the pedestal and assume the wafer is at the same potential. The measurement of this value is rectified to estimate the DC bias on the wafer. However, this solution proves to be inaccurate because of the high level of RF noise proximate the pedestal assembly that penetrates into the electrical measuring circuitry. Additionally, this technique requires substantial modification of the chamber to facilitate coupling a probe and its associated circuitry to the pedestal.
Therefore, there exists a need for a device that can estimate DC bias voltage at a wafer during processing without experiencing the limiting and erroneous effects of RF noise and/or physical degradation of voltage probes that contact the plasma.
The disadvantages heretofore associated with the prior art are overcome by an inventive probe for measuring voltage on a wafer located in a process chamber. The inventive probe comprises a conductive member embedded in a dielectric material. The dielectric material is in contact with a wall of the process chamber and exposed to the interior of the chamber. The dielectric material can be quartz, alumina and the like. The probe has an electrode fabricated from a conductive material, preferably a metal such as copper. The probe has a conductor that can coupled to a measuring instrument, such as a voltmeter, for measuring the voltage levels detected by the conductive member.
The inventive probe in one embodiment utilizes conductive member, i.e., an electrode, embedded in an outside wall of the chamber, e.g., mounted in a quartz window in the chamber wall. From such a location, the electrode receives the RF voltage oscillations of the plasma and couples these voltage oscillations to measuring equipment. By making certain circuit analysis assumptions, the value coupled to the measuring equipment represents an estimated DC bias voltage on the wafer. Such a probe configuration enjoys the advantage of low noise levels for good signal accuracy, no particle generation and can be easily adapted into systems currently in use.